Jennifer A. Clack

Lower jaws, lower tetrapods–a review based on the Devonian genus Acanthostega

The lower jaw of the Devonian tetrapod Acanthostega is described for the first time. Redescriptions are provided for the lower jaws of the elpistostegid Panderichthys , the Devonian tetrapods Elginerpeton, Obruchevichthys, Metaxygnathus, Ventastega and Ichthyostega , and the Carboniferous tetrapods Crassigyrinus, Megalocephalus and Gephyrostegus . The character distri- butions thus revealed differ considerably from previous accounts, particularly in the wide distribution of certain primitive characters. Meckelian ossification in the middle part of the jaw is widespread among Devonian tetrapods, being demonstrably absent only in Acanthostega. Among Carboniferous tetrapods, a tooth-bearing parasymphysial plate is shown to be present in Crassigyrinus and Megalocephalus (having already been demonstrated by other authors in Whatcheeria and Greererpeton). A phylogenetic analysis of 26 early tetrapods including all the aforementioned genera, scored for 51 lower jaw characters, produces at least 2,500 equally parsimonious trees. However, the lack of resolution lies largely in a big top end polychotomy containing anthracosaurs, temnospondyls, seymouriamorphs, microsaurs and a nectridean-amniote clade. Below this polycho- tomy, which may correspond approximately to the tetrapod crown group, there is a well-resolved stem-group containing, in descending order, Megalocephalus, Greererpeton, Crassigyrinus , (jaws associated with) Tulerpeton, Whatcheeria, Acanthostega, Metaxygnathus, Ichthyostega, Ventastega and Metaxygnathus (unresolved), an Elginerpeton-Obruchevichthys clade, and Panderichthys. This conflicts with recently published phylogenies by Coates and Lebedev & Coates, which place Tulerpeton and all post-Devonian tetrapods in the amphibian or amniote branches of the tetrapod crown group.

Three-dimensional limb joint mobility in the early tetrapod Ichthyostega

The origin of tetrapods and the transition from swimming to walking was a pivotal step in the evolution and diversification of terrestrial vertebrates. During this time, modifications of the limbs—particularly the specialization of joints and the structures that guide their motions—fundamentally changed the ways in which early tetrapods could move. Nonetheless, little is known about the functional consequences of limb anatomy in early tetrapods and how that anatomy influenced locomotion capabilities at this very critical stage in vertebrate evolution. Here we present a three-dimensional reconstruction of the iconic Devonian tetrapod Ichthyostega and a quantitative and comparative analysis of limb mobility in this early tetrapod. We show that Ichthyostega could not have employed typical tetrapod locomotory behaviours, such as lateral sequence walking. In particular, it lacked the necessary rotary motions in its limbs to push the body off the ground and move the limbs in an alternating sequence. Given that long-axis rotation was present in the fins of tetrapodomorph fishes, it seems that either early tetrapods evolved through an initial stage of restricted shoulder and hip joint mobility or that Ichthyostega was unique in this respect. We conclude that early tetrapods with the skeletal morphology and limb mobility of Ichthyostega were unlikely to have made some of the recently described Middle Devonian trackways.

The axial skeleton of the Devonian tetrapod Ichthyostega.

Ichthyostega was the first Devonian tetrapod to be subject to a whole-body reconstruction. It remains, together with Acanthostega, one of only two Devonian tetrapods for which near-complete postcranial material is available. It is thus crucially important for our understanding of the earliest stages of tetrapod evolution and terrestrialization. Here we show a new reconstruction of Ichthyostega based on extensive re-examination of original material and augmented by recently collected specimens. Our reconstruction differs substantially from those previously published and reveals hitherto unrecognized regionalization in the vertebral column. Ichthyostega is the earliest vertebrate to show obvious adaptations for non-swimming locomotion. Uniquely among early tetrapods, the presacral vertebral column shows pronounced regionalization of neural arch morphology, suggesting that it was adapted for dorsoventral rather than lateral flexion.

Devonian climate change, breathing, and the origin of the tetrapod stem group

The diversification of the tetrapod stem group occurred during the late Middle through the Late Devonian, that is from the Givetian to Famennian stages about 385-365 million years ago. The relationships between the known taxa representing this radiation have currently reached a reasonable consensus so that interpretations of the order of appearance of tetrapod characters is possible. The immediate fish relatives of the earliest limbed tetrapods show what is interpreted as a progressive increase in the spiracular chamber and its opening to the outside. Here, this is inferred to be associated with an increased capacity for air-breathing. Lungs are thought to have been present in most early bony fishes, and were most likely ventilated by air-gulping. This could have brought about a facultative capacity for air-breathing, which the tetrapod stem group exploited to the greatest degree. These adaptations are shown not only in freshwater forms but also in estuarine and marginal marine forms. Estimates of oxygen levels during this period suggest that they were unprecedentedly low during the Givetian and Frasnian periods. At the same time, plant diversification was at its most rapid, changing the character of the landscape and contributing, via soils, soluble nutrients, and decaying plant matter, to anoxia in all water systems. The co-occurrence of these global events may explain the evolution of air-breathing adaptations in at least two lobe-finned groups, contributing directly to the rise of the tetrapod stem group. In contrast to recent studies, low atmospheric oxygen is not considered to be a causal factor in the lack of fossils documenting the evolution of Early Carboniferous tetrapods.

A uniquely specialized ear in a very early tetrapod

The Late Devonian genus Ichthyostega was for many decades the earliest known tetrapod, and the sole representative of a transitional form between a fish and a land vertebrate. However, despite being known since 1932 (ref. 1) from a large collection of specimens, its morphology remained enigmatic and not what was expected of a very primitive tetrapod. Its apparent specializations led it to be considered as a “blind offshoot” or “sidebranch” off the tetrapod family tree, and recent cladistic analyses have disagreed about its exact phylogenetic position within the tetrapod stem group. In particular, its braincase and ear region defied interpretation, such that conventional anatomical terms seemed inapplicable. Using new material collected in 1998 (ref. 9), preparation of earlier-collected material, and high-resolution computed tomography scanning, here we identify and interpret these problematic anatomical structures. They can now be seen to form part of a highly specialized ear, probably a hearing device for use in water. This represents a structurally and functionally unique modification of the tetrapod otic region, unlike anything seen in subsequent tetrapod evolution. The presence of deeply grooved gill bars as in its contemporary Acanthostega suggest that Ichthyostega may have been more aquatically adapted than previously believed.

Ventastega curonica and the origin of tetrapod morphology

The gap in our understanding of the evolutionary transition from fish to tetrapod is beginning to close thanks to the discovery of new intermediate forms such as Tiktaalik roseae. Here we narrow it further by presenting the skull, exceptionally preserved braincase, shoulder girdle and partial pelvis of Ventastega curonica from the Late Devonian of Latvia, a transitional intermediate form between the ‘elpistostegids’ Panderichthys and Tiktaalik and the Devonian tetrapods (limbed vertebrates) Acanthostega and Ichthyostega. Ventastega is the most primitive Devonian tetrapod represented by extensive remains, and casts light on a part of the phylogeny otherwise only represented by fragmentary taxa: it illuminates the origin of principal tetrapod structures and the extent of morphological diversity among the transitional forms.

Earliest Carboniferous tetrapod and arthropod faunas from Scotland populate Romer's Gap

Devonian tetrapods (limbed vertebrates), known from an increasingly large number of localities, have been shown to be mainly aquatic with many primitive features. In contrast, the post-Devonian record is marked by an Early Mississippian temporal gap ranging from the earliest Carboniferous (Tournaisian and early Viséan) to the mid-Viséan. By the mid-Viséan, tetrapods had become effectively terrestrial as attested by the presence of stem amniotes, developed an essentially modern aspect, and given rise to the crown group. Up to now, only two localities have yielded tetrapod specimens from the Tournaisian stage: one in Scotland with a single articulated skeleton and one in Nova Scotia with isolated bones, many of uncertain identity. We announce a series of discoveries of Tournaisian-age localities in Scotland that have yielded a wealth of new tetrapod and arthropod fossils. These include both terrestrial and aquatic forms and new taxa. We conclude that the gap in the fossil record has been an artifact of collection failure.

Devonian tetrapod trackways and trackmakers; a review of the fossils and footprints

Abstract The earliest tetrapods are known from the Upper Devonian. Their remains are becoming better known from increasing numbers of specimens, localities, environments and ichnofossils. Each of the eight (or possibly nine) genera now represented by skeletal fossils is reviewed in its sedimentological, faunal and stratigraphic context, with an assessment of what might be inferred about the habitus and locomotory capabilities of each. Fossil trackways and their interpretations are then re-examined in the context of the known body forms, and consideration given to the degree of fit between the skeletal fossils, the trackways and their interpretations. The currently known Devonian tetrapods are unlikely to have made any of the known tracks, unless they were produced under water. Neither the skeletal fossils nor the trackways show good evidence of terrestrial locomotion among Devonian tetrapods. When the fossil material and recent phylogenetic analyses are taken in combination, it appears that neither tetrapods nor limbs with digits are likely to have arisen before the Frasnian. This should be borne in mind in palaeoecological studies of these animals.

Comparative axial morphology in pinnipeds and its correlation with aquatic locomotory behaviour

Regional variation in the axial skeleton of pinnipeds (seals and walruses) and its correlation with aquatic locomotory behaviour is examined using vertebral functional profiles. The results demonstrate clear morpho‐functional differences in the thoracolumbar region of modern pinnipeds (Phocidae, Otariidae, Odobenus) that can be strongly linked to swimming style. Phocid seals have a rigid thoracic region attached to a highly flexible lumbar region with long muscular lever arms providing the necessary mobility and leverage to perform pelvic oscillations. Conversely, otariid seals have extremely flexible inter‐vertebral joints along the length of the column which should enhance manoeuvrability and turning performance. They also have greater muscular leverage in the anterior thoracic region to support pectoral oscillations. Odobenus (walrus) shows vertebral characteristics most similar to phocids, but with some otariid qualities, consistent with an intermediate or mixed form of aquatic locomotion, with pelvic oscillation dominating over pectoral oscillation. Comparison of the vertebral functional profiles in the fossil taxon Allodesmus kernensis with those of modern pinniped clades reveals that this extinct pinniped may also have used a combination of pectoral and pelvic oscillatory movements during swimming, but in a manner opposite to that of Odobenus, with pectoral oscillatory movements dominating. This study raises questions about the evolution and diversification of pinniped locomotory behaviours, but also provides the necessary framework to begin to examine axial mechanics and locomotory stages in other fossil pinnipedimorphs and their relatives in more detail.

Palaeontology: A firm step from water to land

A project designed to discover fossils that illuminate the transition between fishes and land vertebrates has delivered the goods. At a stroke, our picture of that transition is greatly improved.When fins became limbsThe transition between fishes and limbed vertebrates, or tetrapods, occurred over 370 million years ago and required changes to virtually the entire body. Sensational fossil finds, and reinterpretations of old ones, have radically altered thinking on this topic in the past 20 years. But the transition itself – the very point where fishes became tetrapods – remains obscure. What fossils there are tend to be incomplete or badly preserved. All that changes with the discovery of remarkable new fossils from the late Devonian of Canada of a near-complete transitional form preserved in the round. Its a fish with fins, but fins that flexed and extended like arms and hands. It has tetrapod-like ribs, a mobile neck and wrist. The impact of this discovery will be felt far and wide in evolutionary biology. On the cover, the fossil as found emerges from under a log as it might have in life in a shallow-water habitat.